Allow to cancel a reconciled expense

[Matth7878]

Sometimes I reconcile movement but make an error and don't notice it when I confirm reconciliation.

Other times a movement appears as pending and I reconcile it. But pending movement disappear from bank statement afterward and finally appears again later.

For both case it would be nice to unreconcile an expense but UI won't let you change state once you mark it as reconciled. It would be much better if it was possible to unmark an expense a reconciled instead to have to duplicate it and delete it. As it is something a bit uncommon I would suggest to only add an entry in popup menu.